Buggy code:
public class MobString { private ArrayList < Sheep > sheepArray1 = new ArrayList < Sheep > ( ) ; private ArrayList < Wolf > wolfArray1 = new ArrayList < Wolf > ( ) ; private ArrayList < Witch > witchArray1 = new ArrayList < Witch > ( ) ; private ArrayList < Ocelot > ocelotArray1 = new ArrayList < Ocelot > ( ) ; private ArrayList < Pig > pigArray1 = new ArrayList < Pig > ( ) ; private ArrayList < Cow > cowArray1 = new ArrayList < Cow > ( ) ; private ArrayList < Skeleton > skellyArray1 = new ArrayList < Skeleton > ( ) ; private ArrayList < Bat > batArray1 = new ArrayList < Bat > ( ) ; public String mobOfChoice1 = "wolf" ; public int numberMobs1 = 2 ; private ArrayList < Sheep > sheepArray2 = new ArrayList < Sheep > ( ) ; private ArrayList < Wolf > wolfArray2 = new ArrayList < Wolf > ( ) ; private ArrayList < Witch > witchArray2 = new ArrayList < Witch > ( ) ; private ArrayList < Ocelot > ocelotArray2 = new ArrayList < Ocelot > ( ) ; private ArrayList < Pig > pigArray2 = new ArrayList < Pig > ( ) ; private ArrayList < Cow > cowArray2 = new ArrayList < Cow > ( ) ; private ArrayList < Skeleton > skellyArray2 = new ArrayList < Skeleton > ( ) ; private ArrayList < Bat > batArray2 = new ArrayList < Bat > ( ) ; public String mobOfChoice2 = "no<seq2seq4repair_space>mob" ; public int numberMobs2 = 0 ; private ArrayList < Sheep > sheepArray3 = new ArrayList < Sheep > ( ) ; private ArrayList < Wolf > wolfArray3 = new ArrayList < Wolf > ( ) ; private ArrayList < Witch > witchArray3 = new ArrayList < Witch > ( ) ; private ArrayList < Ocelot > ocelotArray3 = new ArrayList < Ocelot > ( ) ; private ArrayList < Pig > pigArray3 = new ArrayList < Pig > ( ) ; private ArrayList < Cow > cowArray3 = new ArrayList < Cow > ( ) ; private ArrayList < Skeleton > skellyArray3 = new ArrayList < Skeleton > ( ) ; private ArrayList < Bat > batArray3 = new ArrayList < Bat > ( ) ; public String mobOfChoice3 = "no<seq2seq4repair_space>mob" ; public int numberMobs3 = 0 ; private ArrayList < Sheep > sheepArray4 = new ArrayList < Sheep > ( ) ; private ArrayList < Wolf > wolfArray4 = new ArrayList < Wolf > ( ) ; private ArrayList < Witch > witchArray4 = new ArrayList < Witch > ( ) ; private ArrayList < Ocelot > ocelotArray4 = new ArrayList < Ocelot > ( ) ; private ArrayList < Pig > pigArray4 = new ArrayList < Pig > ( ) ; private ArrayList < Cow > cowArray4 = new ArrayList < Cow > ( ) ; private ArrayList < Skeleton > skellyArray4 = new ArrayList < Skeleton > ( ) ; private ArrayList < Bat > batArray4 = new ArrayList < Bat > ( ) ; public String mobOfChoice4 = "no<seq2seq4repair_space>mob" ; public int numberMobs4 = 0 ; public MobString ( WaterGunListener wgl ) { } public void cowboy ( Player p , Ghast g , Location loc ) { } private boolean mobStringExists ( int stringNum ) { } private void createMobs ( Player p , Ghast g , Location loc ) { } private void emptyAllArrays ( ) { sheepArray1 = new ArrayList < Sheep > ( ) ; wolfArray1 = new ArrayList < Wolf > ( ) ; witchArray1 = new ArrayList < Witch > ( ) ; ocelotArray1 = new ArrayList < Ocelot > ( ) ; pigArray1 = new ArrayList < Pig > ( ) ; cowArray1 = new ArrayList < Cow > ( ) ; skellyArray1 = new ArrayList < Skeleton > ( ) ; batArray1 = new ArrayList < Bat > ( ) ; sheepArray2 = new ArrayList < Sheep > ( ) ; wolfArray2 = new ArrayList < Wolf > ( ) ; witchArray2 = new ArrayList < Witch > ( ) ; ocelotArray2 = new ArrayList < Ocelot > ( ) ; pigArray2 = new ArrayList < Pig > ( ) ; cowArray2 = new ArrayList < Cow > ( ) ; skellyArray2 = new ArrayList < Skeleton > ( ) ; batArray2 = new ArrayList < Bat > ( ) ; sheepArray3 = new ArrayList < Sheep > ( ) ; wolfArray3 = new ArrayList < Wolf > ( ) ; witchArray3 = new ArrayList < Witch > ( ) ; ocelotArray3 = new ArrayList < Ocelot > ( ) ; pigArray3 = new ArrayList < Pig > ( ) ; cowArray3 = new ArrayList < Cow > ( ) ; skellyArray3 = new ArrayList < Skeleton > ( ) ; batArray3 = new ArrayList < Bat > ( ) ; sheepArray4 = new ArrayList < Sheep > ( ) ; wolfArray4 = new ArrayList < Wolf > ( ) ; witchArray4 = new ArrayList < Witch > ( ) ; ocelotArray4 = new ArrayList < Ocelot > ( ) ; pigArray4 = new ArrayList < Pig > ( ) ; cowArray4 = new ArrayList < Cow > ( ) ; skellyArray4 = new ArrayList < Skeleton > ( ) ; <START_BUG> batArray3 = new ArrayList < Bat > ( ) ; <END_BUG> } private Sheep getFinalSheepObject ( int whichTier ) { } private Wolf getFinalWolfObject ( int whichTier ) { } private Witch getFinalWitchObject ( int whichTier ) { } private Ocelot getFinalOcelotObject ( int whichTier ) { } private Pig getFinalPigObject ( int whichTier ) { } private Cow getFinalCowObject ( int whichTier ) { } private Skeleton getFinalSkeletonObject ( int whichTier ) { } private Bat getFinalBatObject ( int whichTier ) { } }

Fix:
batArray4 = new ArrayList < Bat > ( ) ;